The proposed studies address very basic questions regarding interactions between motoneurons and the muscle fibers they innervate (i.e., neuron-target cell interaction). Activity (or inactivity) dependent plasticity is well described in a variety of neural systems, including diaphragm muscle (DIAm) motor units where altered use leads to changes at muscle fiber, neuromuscular junctions (NMJs), and phrenic motoneurons. In previous studies, we found that when DIAm motor units are inactivated by spinal hemisection (SH) at C2, neuromuscular transmission is preserved and DIAm fibers show little adaptation. In contrast, when DIAm paralysis is induced by unilateral tetrodotoxin (TTX) nerve blockade, susceptibility to neuromuscular transmission failure dramatically increases, and there are profound alterations in the morphology and mechanical properties of DIAm fibers. A major difference between these two models is the matching of neuron-target cell activity. Following SH, phrenic motoneurons are inactive, whereas phrenic motoneurons remain active after TTX nerve blockade. By comparing plasticity induced by SH versus TTX, the confounding influence of matching (SH) versus mis-matching (TTX) target cell activity can be examined. [unreadable] [unreadable] The proposed studies will focus on the role of neurolrophins (BDNF and NT4/5) in preserving neuron-target cell interactions following SH. Our working hypotheses are that a) presynaptic vesicle release and cycling at NMJs is modulated by neurotrophins (BDNF and NT4/5), and that this effect is dependent on motor unit type; b) SH and TTX induced plasticity alters synaptic vesicle release and cycling and its modulation by neurotrophins but in opposite ways; c) neurotrophin expression at phrenic motoneurons varies with motoneuron size and cFOS expression (both reflections of motor unit type); and d) motoneuron inactivity (SH) leads to an increase in neurotrophin expression to counter the effects of presynaptic inactivity. [unreadable] [unreadable] Five specific aims are proposed: Specific Aim 1: To determine the influence of neurotrophins on NMJ synaptic vesicle release and cycling at different DIAm motor unit types. Specific Aim 2: To determine whether NMJ plasticity induced by SH and TTX alters synaptic vesicle release and cycling and its modulation by neurotrophius. Specific Aim 3: To determine whether basal levels of BDNF and NT4/5 in phrenic motoneurons vary across motor unit types. Specific Aim 4: To determine whether phrenic motoneuron inactivity following SH increases BDNF and NT4/5 expression in motoneurons, and whether this response varies with motoneuron size. Specific Aim 5: To determine whether the restoration of phrenic motoneuron activity after SH (crossed-phrenic phenomenon) alters synaptic vesicle release and cycling at different DIAm motor unit types and/or BDNF and NT4/5 expression at reactivated phrenic motoneurons. [unreadable] [unreadable]